The present invention relates to an image forming method for forming each of toner images of different colors on a respective photoconductive drum or similar image carrier, transferring the toner images to an intermediate transfer belt, and transferring them from the belt to a recording medium, and an apparatus for practicing the same.
A color copier, color facsimile apparatus, color printer or similar color image forming apparatus is extensively used today. Some different types of image forming apparatuses are known in the art, as follows.
(1) A color image forming apparatus includes a plurality of photoconductive drums arranged around a single image transfer drum. Color images formed on the photoconductive drums are directly transferred to a paper or similar recording medium wrapped around the image transfer drum. For example, latent images respectively formed on the photoconductive drums are developed by two developing devices, as taught in Japanese Patent Laid-Open Publication No. 2-12273 by way of example. Specifically, a first developing unit assigned to a first photoconductive element has a yellow toner and a cyan toner developing section. A second developing unit assigned to a second photoconductive element has a magenta toner and a yellow toner developing section. Color images formed on the photoconductive drums by the first and second developing devices are transferred to a paper wrapped around the image transfer drum. This type of apparatus as the following problem. The first and second photoconductive drums are spaced from each other. Therefore, if either of the two photoconductive drums are arranged around the transfer drum with the same orientation, they each contact the transfer drum at a different position (image transfer position). Consequently, the first developing device including a first charger in addition to the first photoconductive drum and developing device and the second developing device including a second charger in addition to the second photoconductive drum and developing device are not replaceable with each other and must be prepared independently of each other.
(2) A color image forming apparatus includes a plurality of image forming devices arranged side by side along an image transfer belt which supports and conveys a paper, as shown in FIG. 6 of the above Laid-Open Publication No. 2-12273. The problem with this type of apparatus is that means for causing the transfer belt to support the paper is required, and in addition only papers of particular sizes corresponding to the transfer belt are usable.
(3) A color image forming apparatus includes four developing devices arranged around a part of a single photoconductive drum, as disclosed in, e.g., Japanese Patent Laid-Open Publication No. 8-160697. The four developing devices each develops a respective latent image formed on the drum. The resulting toner images are transferred to an intermediate transfer belt partly contacting the drum. Subsequently, the toner images are transferred from the belt to a paper. The problem with this type of apparatus is that although the four developing devices arranged around the drum may be identical in configuration, they each is held in a particular position with respect to the drum. As a result, such developing devices are not replaceable with each other. In addition, the number of copies or printings available with this apparatus for a unit time is limited.
(4) A color image forming apparatus includes a single image transfer belt and four photoconductive drums. A single developing unit is assigned to each of the four photoconductive drums. Color images transferred from the photoconductive drums to the image transfer belt are transferred to a paper. Although a relatively high image forming speed is achievable with this type of apparatus, one image forming unit including a charger must be assigned to each of the drums, scaling up the entire apparatus.